Darksteel Monolith

Eight mana for a rock that produces no mana is a joke on paper, which is precisely the tension the card is built around: the sticker price is not what you pay to profit from it. Once resolved (and it resolves for good, since the usual battlefield sweepers cannot touch it), it becomes a colorless-spell discount engine, letting one colorless spell per turn cast from your hand hit the stack for regardless of its printed cost. That net catches far more than artifacts: Eldrazi titans, colorless planeswalkers, colorless instants and sorceries all qualify, so the deck it wants is one packed with heavy colorless payoffs across the type chart. The eight is a toll you pay once to open a spigot, and the value math turns on how many bombs you can chain through the free window over the following turns. It is a deliberate inversion of the mana-rock template: rather than ramping you toward expensive threats, it simply refuses to charge you for one each turn. The indestructibility is not incidental. A cost-reduction piece that dies to a Vandalblast is a liability; making the Monolith hard to remove protects the mana you sank into deploying it, so the engine keeps humming past the point where a fragile enabler would have been picked off. The design lives or dies by the density of colorless targets behind it, which is the same discipline any enabler-first build demands.